Many systems in homes and in automobiles are operated remotely using low-power radio transmitters, some of which comply with FCC Part 15 rules. Examples of remotely operated systems include garage door openers and intruder alarm disable switches. Although these systems, like modern cordless telephones, employ some basic security encoding of their transmitted signals, they have some pitfalls. For example, the basic security coding on remotely operated systems may be easily duplicated.
Typically, remotely operated systems include a transmitter and a receiver, each having the same security code programmed onto it. The security code is identical on each transmission between the transmitter and receiver, is transmitted at a low rate, and has a very limited number of separate code combinations possible (typically up to 2.sup.15). This basic security code is easily duplicated by intercepting a transmission from the transmitter, or by "tumbling" the security code in a duplicate transmitter.
In addition to the problem of transmitter duplication, there is typically no verification of the transmitter by the receiver. Thus, if a duplicate or pirate transmitter is created in one of the aforementioned ways, there is no way for the receiver to distinguish the duplicate transmitter from a legitimate transmitter by interrogation, because the radio link operates in one direction only.
Some improved transmitters have been designed to attempt to overcome these defects by using a "rolling code" which changes each time the transmitter is successfully used. However, these improved transmitters are still vulnerable to duplication because there is no bi-directional verification of the transmitter. This situation is similar to that which currently exists in cellular systems, where duplication of a mobile station's identification number (MIN) and its electronic serial number (ESN) in a "pirate" mobile station allows calls to be made on the pirate mobile station which are then charged to the account of a legitimate user.